Fall and winter are traditionally boom times for respiratory viruses-a point well proven by this year’s confluence of influenza, RSV, and COVID-19. Almost 9 million people nationwide have been sickened by the flu already this season, RSV is surging among children, and COVID-19 continues to infect tens of thousands of people in the U.S. each day.

But why does cold weather typically translate to cold and flu season? Experts often point to changes in human behavior-namely that chilly temperatures force people inside, where it’s easier for germs to spread. But a new study published in The Journal of Allergy and Clinical Immunology suggests another explanation can be found within our noses.

As the body’s gateway to the outside world, the human nose is equipped with defenses meant to stop invaders, such as viruses and bacteria, in their tracks. In 2018, researchers from Boston’s Northeastern University, Massachusetts Eye and Ear (a teaching hospital affiliated with Harvard Medical School), and several other organizations described one of those shields in a paper also published in The Journal of Allergy and Clinical Immunology. When the nose detects bacteria, they found, it releases a swarm of tiny fluid-filled sacs meant to attack and neutralize it.

“When you kick a hornet’s nest, the hornets swarm out and try to kill whatever the attacker is before it can attack the nest,” says co-author Dr. Benjamin Bleier, director of otolaryngology translational research at Massachusetts Eye and Ear. “That’s what the body’s doing.”

In the new study, Bleier and his collaborators show that the nose deploys similar defenses against common respiratory viruses, including two rhinoviruses (frequent causes of the common cold) and a coronavirus (though not the one that causes COVID-19). They also set out to answer another question: does cold weather dampen the effectiveness of the nose’s natural immune response?

Previous research suggests common respiratory viruses thrive at lower temperatures. In 2015, a research team from Yale University used mice to demonstrate that rhinoviruses are better at replicating in colder temperatures because antiviral immune responses aren’t as strong under these conditions. Research from the National Institutes of Health has also shown that influenza viruses are better at spreading in chilly climates.

The new paper builds on that earlier work by laying out a “true, quantitative, biologic explanation…of why the body is more susceptible to viral infections when being exposed to cold,” Bleier says.

For the study, researchers measured the nasal temperatures of healthy human volunteers at about 74°F versus approximately 40°F. They found that the nasal cavity’s temperature dropped by about 9°F in the colder condition. Next, in the laboratory, they exposed nasal cell samples to a similar reduction in temperature, to mimic what actually happens within the nose in a cold climate. They found that the immune response was significantly blunted at the lower temperature.

In addition to helping explain long-observed trends about respiratory viruses spiking during cold months, the finding could eventually lead to new therapeutics, says study co-author Mansoor Amiji, chairman of the department of pharmaceutical sciences at Northeastern. If researchers can find a way to boost the nose’s innate immune response, even under cold temperatures, they could feasibly stop more viral illnesses before they take root.

“You only need to trigger that response locally to protect the rest of the body,” Bleier says. “It just speaks to how important the nasal cavity is.”